This invention relates to an automated machine for assembling a heat exchanger having perforated plate fins interposed between a pair of perforated tube sheets and being interlaced with hairpin tubes and, in particular, to lacing and expanding hairpin tubes into the plate-fin coil.
In U.S. Pat. No. 4,228,573 which issued to Barnard, there is disclosed an automated machine for manufacturing fin plate heat exchangers of the type suitable for use in air conditioning systems. Each unit, as it is being assembled, is retained within a holding fixture with the tube receiving holes of the retained fin pack being supported in a generally horizontal position. Initially, the perforated fins and one perforated tube sheet or header are placed in the fixture and then clamped in place using a pair of jaws that are adapted to swing upwardly into locking engagement against the perforated end faces of the partially assembled unit. The fixture is then passed through a series of processing stations wherein the hairpins are laced into the package, a second header is inserted over the laced tube ends and finally the tubes are expanded into locking contact against the surrounding elements.
The fixtures containing the retained units in the Barnard device are carried between processing stations on a conveyor belt. At each processing station, the fixture is lifted from the belt and carried to a remote position wherein a specific manufacturing step is carried out. Upon the completion of the operation, the fixture is again returned to the conveyor belt and moved to the next subsequent station. The timing of the various operational steps in Barnard is not coordinated and, in order to avoid bottlenecks along the line, fixtures containing partially assembled units are moved to temporary overhead storage areas where they are held until needed. As a consequence, each fixture must be loaded and unloaded from the main conveyor a number of times before the assembly operation is completed. This excessive handling slows down the automated process and raises the unit cost of the assembled units. The apparatus needed to carry out the many handling and processing steps is also relatively complex and difficult to maintain.
A further disadvantage associated with the Barnard machine concerns the clamping mechanism used to hold the units within each of the fixtures. The clamps are arranged to act upon the perforated end faces of the units and thus prevent access to these critical areas during the manufacturing process. During a number of assembly operations, the clamps must be lowered so that the operation can be completed. This, of course, releases the unit from the fixture at a time when it is being worked upon. As a consequence the fin plates can become misaligned or damaged and, as a consequence, the unit may eventually have to be scrapped.
It should be noted that in Barnard the hairpin tubes are laced into the fin packs in a row by row progression. Each row of tubes is horizontally disposed with a tray so that the legs of the tubes are aligned with the tube receiving holes formed in the unit. The hairpin tubes are then pushed from the tray through the fin pack unit with the aid of lacing rods at a first processing station and the unit moved into a second processing station where the tubes are expanded. In the event the heat exchanger unit requires more than one row of hairpins, extra lacing and expanding stations must be added to the machine to accommodate each additional row. In an assembly having four rows of hairpins, for example, four lacing and expanding stations would be needed to fully lace a single unit. Clearly, this large number of processing stations consumes a great deal of valuable floor space and requires the utilization of large amounts of redundant tooling.
Lastly, it should be pointed out that the tube expansion process practiced by Barnard produces non-uniform changes in the length of the tube legs. Upon lacing the hairpins into the fin pack, Barnard firmly supports the bend end of the tubes firmly against movement and then drives an expanding tool upwardly through the open end of the tubes. Under the action of the tool, the metal in the tubular legs is compressed axially as it is expanded radially. The tube legs are thus caused to shrink in length and this shrinkage may be non-uniform. As a consequence the length of the tubes between the bottom tube sheet and the end of the tubes varies between tubes. This, in turn, can cause misalignment problems and the like when the tube ends are later belled and the return bends are mounted therein.